I'm afraid I don't know much if anything useful about Cubic and what it provides as standard.
If they have an option for you to purchase data packages, that might suggest that VW are now using this rather than their previous Car Stick to provide a built in 3G/4G internet access but the owner has to purchase the data allowance. Do you have any details of prices, data amounts and expiry ?
It is obviously worth comparing the cost of that vs either a dedicated dongle or using your phone. It depends how much internet access you would use and what for. I understand internet music and radio may be available, but it could be a high price just to use the integrated VW system.
Personally the dongle works well for me. It is permanently connected to one of the USB ports, so it doesn't use a battery or require charging and is always available and completely seemless regardless of who is driving and it can provide internet access for passengers. The upfront cost of my dongle was ~ ÂŁ40 and I got a free Three data SIM which has 200 MB of free data every month. I've never had to top it up, so I haven't paid a penny for data over 18 months, but I don't use the internet much in the car and it is mostly for the extra features it provides for Sat Nav and Car Net.
I believe tethering works better with some phones than others and may suit if you already have a data bundle.